Asphaltene-containing residual oil is a residue by-product of refineries that process crude oil into economically valuable light hydrocarbons, such as gasoline, and of coal hydrogenation plants that convert coal into liquid fuels. Residual oil is a heavy, viscous hydrocarbon unsuitable for conventional refinery processing by hydrodesulfurization, hydrocracking, or catalytic cracking because of the excessive amounts of included asphalt and metals. Conventionally, residual oil is further processed in a solvent-deasphalting plant by contacting a feed stream of residual oil with a solvent such as iso-butane, normal-butane, n-pentane, isohexane, etc. under such conditions of temperature and pressure that the mixture separates into two primary liquid streams: a primary stream of deasphalted oil (DAO) and most of the solvent, and a primary stream of asphaltene and the remainder of the solvent. The solvent in these streams is recovered in a solvent recovery unit for re-use. Although the asphaltene product is of relatively limited value, the DAO product is very valuable because it can be recycled back to a refinery where it is converted into gasoline or the like.
Conventionally, solvent recovery units separately add heat to, and then process, each primary stream in two steps. First, the streams are applied to respective vaporization towers wherein most of the solvent in the heated streams is flashed to a vapor producing respective streams with reduced solvent. Then, the reduced solvent streams are applied to respective strippers, wherein, an inert gas, such as steam, strips the remaining solvent from the reduced solvent streams to produce separate product streams of DAO and asphaltene substantially free of solvent.
Early approaches to adding heat to the primary stream of asphaltene and solvent are described in U.S. Pat. Nos. 2,943,050, 3,423,308, and 4,017,383. These patents disclose applying this primary stream to a furnace heated by a flame. This approach proved to be unsatisfactory because, in order to raise the temperature of the stream to a value at which solvent recovery in a vaporization tower can be effected, the temperature of the furnace walls at many locations approached asphalt decomposition temperature. As an alternative arrangement, the primary stream of asphaltene and solvent was indirectly heated with hot oil flowing in a closed loop. However, this arrangement added significantly to the cost of a deasphalting unit because decomposition or contamination of the hot oil occurred over a period of time.
These problems are overcome using the expedient disclosed in U.S. Pat. No. 4,395,330 wherein a portion of the stream of reduced solvent DAO produced by a vaporization tower prior to the stripping process is indirectly heated and used to indirectly heat the primary stream of asphaltene and solvent before being admixed with the primary stream of DAO and solvent. A drawback to this expedient is the increased physical size of the DAO recovery circuit, and difficulty in controlling the temperature of the vaporization tower. Absent adequate temperature controls, the vaporization tower is subject to sporadic carry-over of DAO into the solvent. Furthermore, the DAO added to the primary stream of DAO and solvent reduces the efficiency of the vaporization tower in separating solvent from the DAO.
It is therefore an object of the present invention to provide a new and improved process and apparatus for solvent-deasphalting asphaltene-containing residual oil which overcomes the drawbacks of the prior art discussed above.